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Apple released the developer preview of Mac OS X 10.7 Lion yesterday, giving the first peek of what's to come on the Mac. With iOS-like UI design, easier file sharing and system-wide AutoSave with file versioning, as well as a fundamental rethinking of file management and application control, the eighth major revision to Apple's desktop OS will be the most significant upgrade since Tiger was released back in 2005. Lion will be available this summer, but registered Mac Developer Program members can download the preview now.

When Steve Jobs announced Lion last fall, he said it would "combine the power of Mac OS X with the magic of iPad." And, as expected, the interface design borrows a lot of elements from iOS. The Mail widescreen layout looks very much like Mail.app on the iPad, finally bringing the popular side-by-side view to the desktop. The new "Launchpad" view lets users launch apps as they would on iOS, selecting from a Springboard-like grid of icons. Apple has even reversed the direction of scrolling to match iOS: now, when you do a two-finger swipe down on a trackpad, the screen moves down - in the direction of your fingers - rather than up, as it does in current versions of Mac OS X. It even makes a "rubber-band" visual effect when you get to the bottom of the screen. So it's clear Apple's really trying to leverage iOS popularity here.

It's far more than a graphical makeover, though, as Apple has made subtle but significant shifts to fundamental activities like saving files and quitting programs in Lion. Auto Save will be implemented on all 10.7-compatible apps, allowing all open documents to be automatically saved as changes are made. Documents can be locked to prevent accidentally overwriting data, and will be "auto-locked" if they haven't been opened in two weeks. A very interesting new development is the Versions feature: Lion will save snapshots of your documents every hour they are open, and let you browse through previous versions and restore them using a Time Machine-style interface.

Also, the command-Q shortcut will apparently be joining command-S in the Trash Can of history with Lion's Resume feature. In addition to auto-save of documents, Lion now auto-saves program states, meaning that when you come back to a program after you log out or reboot, the windows will all be in the exact same state you left them in. The "glowing dot" indicator on the Dock is gone in Mac OS X 10.7, underscoring graphically the basic concept that - from the user perspective - there's no difference between running and non-running applications. There are just different workspaces that you switch between, in regular windows or with the new Lion full-screen interface (another iOS feature making its way to the Mac).

There's a host of other productivity and functionality improvements. Application sandboxing means that malware will have a harder time gaining control of your system through security holes in programs. AirDrop lets you copy files to other Lion machines over the network, dropping them in the destination Mac's Download folder. You can now switch between different online accounts for Mail, iCal, Address Book and Chat - a boon for people like me with the same computer for work and personal use - using a new "Internet Accounts" preferences pane. The Finder is completely written and sports a new iTunes-like interface that automagically groups files by type. And OS X Server is now bundled with the desktop OS, rather than being a separate product as it is now. With new file-sharing and profile management tools for iDevices, it's likely that many home users as well as businesses will find themselves running Lion servers.

In a break with traditional distribution methods, the developer preview is available through the Mac App Store rather than by download or DVD. The process has been rocky, however: numerous developers have said they could not download the preview during high utilization periods. Some have also reported that they were unable to continue downloads that had been interrupted. While some of these problems are to be expected, it raises concerns if Apple is to switch to an App-Store-only model of distributing upgrades. Will this mean that you need Mac OS X 10.6.6 to upgrade to Lion? Apple has so far not responded to requests for comment.